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Abstract
The virions of tobacco necrosis virus, Nebraska isolate (TNV-NE), an extremely stable virus, have been dissociated and reassembled. The virions of TNV-NE were dissociated either in 2% phosphotungstic acid (PTA), or in 15 mM EDTA or by dialysis in 10 mM EDTA. High salt or alkaline conditions failed to disrupt TNV-NE virions. This is the only virus, thus far reported, that can be disrupted solely by EDTA. The dissociated virus was reassembled with added nucleic acid (ssRNA, dsRNA, ssDNA, or dsDNA). Successful reassembly required particular concentrations of divalent ions Ca$\sp{2+}$ and Mg$\sp{2+}$. The reassembled particles with dsDNA were nuclease-resistant. The requirement of EDTA for dissociation and the requirement of the divalent ions for reassembly suggest that, unlike TNV strain D, protein-protein interactions reinforced by the bonds formed between divalent metal ions Ca$\sp{2+}$ (and Mg$\sp{2+}$) and coat protein subunits were the main stabilizing forces for TNV-NE virions. Dissociation and reassembly were evaluated by electron microscopy. A partial (40%) nucleotide sequence surrounding the coat protein gene of TNV-NE has been determined. There are at least four open reading frames (ORFs) in the region sequenced. A portion of the putative first ORF may encode a partial (C-terminal) readthrough domain of the putative polymerase. The second and third ORFs encode 10.7 kDa and 6.2 kDa proteins, respectively. The p10.7 and p6.2 ORFs share high amino acid homology with those in TNV-A. The putative coat protein ORF (30.3 kDa) is located at the 3$\sp\prime$-proximal end of the region sequenced. The deduced amino acid sequence of the TNV-NE coat protein is similar to both TNV-A and TNV-D. The homologies between the TNV strains are, respectively, 51% for strains A-NE, 44% for NE-D, and 45% for A-D. This suggests that TNV-NE, TNV-A, and TNV-D should be classified as distinct viruses rather than TNV strains. Some properties of these viruses are discussed in relation to the amino acid sequences of their coat proteins.
